1. Field of the Invention
The invention relates to devices comprising lead zirconate titanate (PZT).
2. Discussion of the Related Art
There is a global interest in developing surface acoustic wave (SAW) devices of high frequency capability, high power durability, and near-zero temperature dependence of frequency, for a variety of applications, including filters, resonators, and delay lines for paging and wireless telephones, mobile switching systems, and global positioning systems. (See, e.g., K. Higaki et al., IEEE MTT-S Digest, Vol. 6, 829 (1997); S. Shikata et al., Diamond and Related Materials, Vol. 2, 1197 (1993); Y. Shibata et al., Jpn. J. Appl. Phys., Vol. 32, L745 (1993); and T. Shiosaki et al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. UFFC-33, No. 3, May 1986, the disclosures of which are hereby incorporated by reference.) A typical SAW device contains a piezoelectric material layer having an interdigital transducer (IDT) formed thereon. The operation frequency, .function., of a SAW device, is dictated by a simple relation: .function.=.nu./.lambda., where, .nu. is SAW velocity in the material and .lambda. is wavelength (as determined by the line and space size of the fingers of the IDT). A gain in the frequency is achieved by increasing the SAW velocity and/or decreasing the line and space size of IDTs.
Because reduction in line and space size is limited by the capabilities of photolithography, some efforts have focused on finding materials that have increased SAW velocity. Diamond has the highest known acoustic wave velocity, and use of a diamond substrate with a piezoelectric material deposited thereon provides an opportunity to improve the velocity characteristics of SAW devices for high frequency uses. Investigation of piezoelectric materials for use in diamond-based SAW devices has focused largely on AlN and ZnO. While these candidate piezoelectric materials possess low elastic wave attenuation and offer high filtering accuracy relative to some ferroelectric candidates, they tend to have relatively poor piezoelectric properties (i.e., piezoelectric coefficients less than 12.times.10.sup.-12 m/V). They also exhibit relatively weak electromechanical coupling, which limits filter bandwidth. In addition, because dipoles in these materials are not capable of being reoriented, the materials must be used in either single crystal or highly textured form. This structural requirement makes thin film growth on substrates such as diamond difficult.
One alternative material that has more recently attracted attention is ferroelectric lead zirconate titanate--Pb(Zr.sub.x Ti.sub.1-x)O.sub.3 (PZT). (See, e.g., A. S. Nickles et al., Integrated Ferroelectrics, Vol. 10, 89 (1995); B. Jaber et al., Sensors and Actuators A, Vol. 63, 91 (1997); and R. Dat et al., Integrated Ferroelectrics, Vol. 9, 309 (1995).) PZT offers several improvements over previously considered piezoelectric materials. For example, the piezoelectric and electromechanical coupling coefficients of PZT are one order of magnitude higher than those of ZnO. Moreover, easier dipole reorientation under an external field and high remanent polarization allow the use of PZT in forms other than single crystal or highly textured.
Unfortunately, PZT exhibits the desired ferroelectric properties only in its perovskite phase, which is difficult to form on a substrate. This difficulty is generally attributed to a lower nucleation barrier for formation of the non-ferroelectric, non-piezoelectric, metastable pyrochlore phase PZT. Thus, PZT forms in its pyrochlore phase much more readily than its perovskite phase. Moreover, the pyrochlore PZT is not readily transformable to perovskite PZT by methods such as a high-temperature anneal. In evaluating this problem, the use of a lead titanate (PT) seeding layer was reported to ease the nucleation of perovskite PZT on a particular substrate--sapphire, when using sol-gel deposition for both the PT and PZT. (See C. K. Kwok and S. B. Desu, J. Mater. Res., Vol. 8, 339 (1993).) Sol-gel deposition, however, is more of a laboratory technique than a feasible commercial fabrication process. For example, the processing sequence of sol-gel thin films is somewhat incompatible with typical device fabrication technology, and the relatively high potential for contamination also weighs against commercial use of sol-gel. In addition, the sol-gel technique does not provide an oriented structure. MgO buffer layers were similarly found to facilitate deposition of perovskite PZT on GaAs and Si substrates. (See A. Masuda et al., J. Crystal Growth, Vol. 158, 84 (1996).).
Methods for forming PZT in its perovskite form, advantageously in an oriented structure, on a variety of substrates, including diamond, are desired.